1. Field of the Invention
This invention relates to a self-operating valve suitable for draining a stationary boat or other vessel of a type having sufficient inbuilt buoyancy to ensure that the vessel floats when in unladen condition with an internal floor level, i.e. bilge, which is higher than the external water level, such vessels being hereinafter referred to as an elevated bilge vessel.
2. Discussion of Prior Art
Some self draining bilge valves are shown for use in elevated bilge vessels which comprise a buoyant ball or flap or a membrane which is held in place aginst an appropriately shaped apertured member in the side or bottom of the vessel by external water pressure. When any water collects inside the vessel, the pressure differential engendered by the internal and external water levels controls the opening and closing of the valve, i.e. a higher internal level, herein described as a positive head, generates a positive internal pressure which forces the valve open. Conversely, the valve is forced by a negative head.
Valves of this type have two major requirements, one being that the valve should close firmly so as to prevent ingress of water when the vessel is so laden as to depress the bilge level below the external water level, i.e. negative head, and the other being that the valve will open freely for draining at minimum possible positive head. These two requirements are conflicting, with the result that the majority of known designs tend to be unsatisfactory in one or the other aspect. Further, the rate of draining of such valves is usually slow, making them unsuitable for use in a vessel in which speedy recovery after sudden swamping is required, e.g. a support pontoon for a floating bridge, which can become heavily but temporarily depressed by an overpassing vehicle. A fast drain rate is also essential for speedy deployment of such pontoons which often become completely waterlogged during launch procedure. A further disadvantage of known design is the difficulty of cleaning them from inside the vessel when they become clogged with flotsam.